You can download all the 3d files to generate build your own locking plates here.

That page also includes the SolidWorks files so you can go into the CAD and see into my decision making process a bit more. The locking plates are part of the Flat-Pack Camera Arm project.

To build a set of locking plates for yourself you'll need:

Materials

• Plywood
• ~1lb Tin-Bismuth Alloy (I got mine from RotoMetals - Alloy #281)
• 1 Pint or more 2-Part Silicone Mold Rubber (I used Smooth-On MoldStar 15, but just about any silicone intended for mold making will hold up to these temperatures - check the material's datasheet to confirm)
• Baby Powder
• Mixing Cup
• Stir Sticks or Tongue Depressors
• Nitrile Gloves
• 8x 0.5" long Drywall or Wood Screws

Tools

• Hand Drill
• 0.5" Drill Bit
• Hot Plate
• Cheap Cast Iron Skillet
• Chip Brush
• Temperature Gun
• Pliers
• Hammer
• Heat Gun (optional)
• 1' x 1' Melamine Board
• Spirit Level
• Digital Scale
• Pilot Drill for Screws
• Heat-Proof Gloves

Molding silicones can tolerate high temperatures (typically 400° F or above). This makes them ideal for low-temp metal casting alloys like the tin-bismuth I'm using here. There are special mixes that can handle much higher temps, but normally silicones won't be able to cast higher temperature melting metals like Aluminum.

I'm using a Tin-Bismuth alloy that is 58% Bismuth and 42% Tin, which gives the metal a melting temperature of 281° F. There are other alloys that will work well with silicones going by the names "stovetop casting alloy", "bullet casting alloy", "low melt fusible alloy", and "fishing tackle weight alloy". Just make certain they melt below your silicone's rated max temp (compare datasheets for your metal and your silicone) and stay away from alloys containing lead. Although lead is relatively benign in most cases, breathing its casting fumes or long term contact that may lead to ingestion is a serious health hazard.

The heat gun is optional in this case because most molds take a few tries to pull a successful casting from. Pouring a hot casting material into a cold mold usually results in a sub-par casting, but if you know you're going to run the mold a few times, there's little harm in taking a crack at a casting to get your eye in. Casting a metal part or two into this mold will heat it up to an ideal temperature for subsequent castings, however you can improve your odds of a great first time casting by using a heat gun to preheat the silicone mold to between 100-115° F before pouring.